An image sensor is a semiconductor device that converts an optical image into an electrical signal, and it largely includes a charge coupled device (CCD) image sensor and a complementary metal oxide semiconductor (CMOS) image sensor.
The CMOS image sensor forms a photodiode and a MOS transistor within a unit pixel to sequentially detect electrical signals of each unit pixel in a switching scheme, implementing an image.
As a design rule is gradually decreased in the CMOS image sensor, the size of the unit pixel is decreased so that light sensitivity may be decreased. In order to improve such a light sensitivity, a microlens is formed on a color filter.
However, owing to an additional structure such as a dielectric layer and a metal wiring existing in an optical path from the microlens to a photodiode, the light sensitivity may be decreased by the diffraction and scattering of light.
The diffraction and scattering of light as mentioned above may cause a problem of degradation of image quality such as a cross talk in the image sensor.
In order to address this problem, the related art has attempted to improve the sensitivity of the image sensor by forming an optical waveguide on the dielectric layer in the pixel region of the image sensor.
However, although light is vertically incident on the central portion of the image sensor, light is actually incident on a cell at the edge portion of the image sensor with a slant of about 20 degrees. Therefore, the structure of a general optical waveguide, which typically has a vertical shape, may not perform properly in waveguiding the slanted light incident on the outer portion.